The establishment of the complex and precise organization of the vertebrate nervous system relies on the proper guidance of neuronal axons to their targets. In the olfactory system of vertebrates, axons of olfactory sensory neurons project to specific glomeruli in the olfactory bulb. The focus of the research outlined in this proposal is to investigate the mechanisms of axon guidance of olfactory sensory neurons to the olfactory bulb in zebrafish. The advantages offered by the olfactory system of zebrafish, namely a stereotyped glomerular map which is invariant between individuals and in vivo imaging in real time of olfactory neuron axon guidance, will facilitate a detailed mechanistic dissection of axon guidance in vivo. The aims of this proposal are: 1) To investigate the formation of the spatial map of projections to the olfactory bulb in zebrafish. The hypothesis that olfactory receptor neurons expressing the same receptor converge to one or a few glomeruli in zebrafish will be tested. 2) To examine the role of olfactory pioneer axons in the proper guidance of olfactory receptor neurons to the olfactory bulb. These studies will provide a characterization of the map of neuronal connectivity in the zebrafish olfactory bulb, which will be valuable for deciphering the coding of olfactory information in vertebrates, and will have practical implications for a comprehensive analysis of the process of axon guidance in the olfactory system.